The water levels detected in Apollo moon rocks and volcanic glasses are in the thousands of parts per million, at most—which explains why analyses of the samples in the late 1960s and early 1970s concluded that the moon was absolutely arid.

"Only in the last decade have instruments become sensitive enough to even analyze water at those kinds of concentrations," said Gary Lofgren, the lunar curator at NASA's Johnson Space Center in Houston, Texas.

Lofgen, who wasn't involved with the new research, called it "solid work" worthy of further investigation.

That team, however, hadn't been able to prove the water hadn't been introduced to the moon rocks on Earth, perhaps through sloppy handling.

One way to determine a water sample's birthplace is to measure the amounts of different hydrogen isotopes inside the water—a technique unavailable to the Nature team, said James Greenwood, a professor of Earth and environmental sciences at Wesleyan University in Middletown, Connecticut.

Isotope measurements can serve as fingerprints. Water from Earth's mantle has a different isotope ratio than water from a comet, for example.

When the Nature study came out, Greenwood was pioneering a technique that allowed him to study the chemical makeups of Martian meteorites. He later applied his method to samples of the mineral apatite, culled from a variety of moon-rock types, to determine the fingerprint of the water molecules inside.

At the presentation, a second team announced the discovery of water in apatite-bearing rocks from one of the moon's mares, dark regions thought to have been formed by ancient lunar lava flows. (See "Volcanoes Rocked Dark Side of the Moon.")

In part by bombarding the mineral with a particle beam from an electron microprobe, the researchers were able to calculate the amounts of the gases fluorine and chlorine within the sample.

Given known formulas for apatite, the amounts of fluorine and chlorine present suggest there's another compound needed to complete the mineral's crystal structure.

The missing molecule, the team concluded, must be hydroxide—a common component of apatite and a byproduct of the breakup of water.

The finding is "one of the first to detect water in a lunar magmatic mineral" and adds to evidence that moon magma, in general, contains trace amounts of water, according to geoscientist Francis McCubbin, who participated in the research.

But, though discoveries of moon water continue to mount, they're really just drops in the bucket, said McCubbin, of the Carnegie Institution for Science in Washington, D.C.

"While there is a lot more water in the moon than we previously thought," he said, "it is still orders of magnitude drier than the Earth and Mars and therefore completely consistent with the last 40 years of lunar sample observation."

Where's Moon Water From?

Going forward, the researchers plan to investigate how water ended up in the moon. The most common guesses center on the moon's earliest days, shortly after it had been created by the collision of a Mars-size object with Earth.

One possibility, according to Wesleyan's Greenwood, is that icy comets hit the molten young moon as it was still solidifying.

Another possibility, said the Carnegie Institution's McCubbin, is that not quite all the water was driven off when chunks of Earth were flung spaceward to form the moon—in other words, the water may be from an ancient version Earth. (See "Where Is Water on Moon From—Volcanoes, Sun ... Earth?")

The tiny amounts of water now being found in lunar rocks are about what you’d expect if water had been depleted during the moon's fiery birth, McCubbin said. "But you are never going to get rid of all of it."